Hexagonal boron nitride powder can, owing to its good thermal conductivity, be used as filler for polymers in applications simultaneously requiring good electrical insulation capability of the filler used. Furthermore, boron nitride powder is also used as sintering powder for hot pressing, for applications in metallurgy. Moreover, hexagonal boron nitride powder is used in cosmetic preparations, as a lubricant, as a parting compound in metallurgy and as raw material for the production of cubic boron nitride.
Hexagonal boron nitride powder is synthesized industrially by nitriding boric acid in the presence of a source of nitrogen. Ammonia can be used as the source of nitrogen, and then usually calcium phosphate is used as the carrier material for the boric acid. An organic source of nitrogen such as melamine or urea can also be reacted under nitrogen with boric acid or borates. Nitriding is usually carried out at temperatures from 800 to 1200° C. The boron nitride then obtained is largely amorphous, and it is also called turbostratic boron nitride. Hexagonal, crystalline boron nitride is produced from amorphous boron nitride at higher temperatures up to about 2100° C. preferably in a nitrogen atmosphere. For this high-temperature treatment, crystallization additives are also added to the amorphous boron nitride.
In the high-temperature treatment, hexagonal boron nitride (hBN) is formed, as primary particles with lamellar morphology. Typical sizes of the lamellae are in the range from approx. 1 to 20 μm, but sizes of the lamellae up to 50 μm or more are also possible. Usually the heat-treated product is ground or deagglomerated after production, to obtain processable powder.
The thermal conductivity of hexagonal boron nitride is greater in the plane of the lamella (a-axis) than perpendicular to it (c-axis). In the direction of the c-axis the thermal conductivity is 2.0 W/mK, but in the direction of the a-axis it is 400 W/mK (see R. F. Hill, SMTA National Symposium “Emerging packaging Technologies”, Research Triangle Park, N.C., Nov. 18-21, 1996).
As well as lamellar boron nitride primary particles or agglomerates of said primary particles, which are formed in the synthesis of hexagonal boron nitride, hexagonal boron nitride powder for uses as filler are also often used in the form of specially produced granules, i.e. in the form of secondary particles formed from the primary particles. Granulation improves the processing properties, such as the free-flowing properties and metering properties, of the boron nitride powder, and higher degrees of filling and higher thermal conductivities can be achieved for example in polymer-boron nitride composites. There are various methods for production of these secondary particles, giving granules with varying morphology and varying properties.
The specially produced granules are often also called “agglomerates”, just as for the agglomerates or aggregates produced in the synthesis of hexagonal boron nitride.